Tapchangers for regulating transformers

ABSTRACT

A tapchanger for a regulating transformer includes a rotatable combined diverter switch tap selector, of which the fixed contacts are arranged in a circular row and are mounted at equal free distances from each other and at least one fixed contact extends over a larger arc of a circle than the other contacts and in which a set of switching contacts mounted for rotation about the axis of the circular row and cooperating with the fixed contacts is adapted to be rotated, for the changeover operation between at least one pair of adjacent fixed contacts, through a greater angle than for the changeover operation between the other pairs of adjacent fixed contacts.

United States Patent [5 4] TAPCHANGERS FOR REGULATING TRANSFORMERS 9 Claims, 5 Drawing Figs.

[52] US. Cl 323/435 R, 200/11 TC [51] Int. Cl l-l02p13/06, l-lOlh 19/58, l-lOlh 21/78 [50] Field of Search 323/435 R; 200/11 TC Primary Examiner-JD. Miller Assistant ExaminerA. D. Pellinen Attorney-Imirie, Smiley, Synder and Butrum ABSTRACT: A tapchanger for a regulating transformer includes a rotatable combined diverter switch tap selector, of which the fixed contacts are arranged in a circular row and are mounted at equal free distances from each other and at least one fixed contact extends over a larger arc of a circle than the other contacts and in which a set of switching contacts mounted for rotation about the axis of the circular row and cooperating with the fixed contacts is adapted to be rotated, for the changeover operation between at least one pair of adjacent fixed contacts, through a greater angle than for the changeover operation between the other pairs of adjacent fixed contacts.

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v"BUR ART Impedance impedance impcd nce INVENTOR ATTORNEY TAPCHANGERS FOR REGULATING TRANSFORMERS The invention relates to a tapchanger for a regulating transformer consisting of or comprising a combined diverter switch tap selector for the fine voltage steps having in each phase a number of fixed contacts arranged in a circular row and appropriated for the connection with the taps of the transformer and at least one set of at least two mechanically rigidly coupled switching contacts cooperating with said fixed contacts and mounted for rotation about the axis of said circular row, one switching contact of said set being directly connected to a point of junction appropriated for the connection with a network-line or star-point conductor and the or each other switching contact of said set lying at a fixed angular distance from the first mentioned switching contact and being connected with said point of junction via a transition impedance, and in which the fee distances between the consecutive fixed contacts are equal and the rotatable switching contacts are positioned at such angulardistances from one another, that, during their rotation past the fixed contacts, they are placed into and out of contact with said fixed contacts one after the other and each time interconnect temporarily two adjacent fixed contacts via an impedance.

In known tapchan gers of this kind the fixed contacts of the combined diverter switch tap selector are equal and they lie with their radial center line at equal angular distances. This means, that each changeover operation between two adjacent fixed contacts takes place through the same switching angle and in all stationary positions of the rotatable switching contacts, in which positions the switching contact connecteddirectly with the point of junction is in contact with a fixed contact, the smallest insulating distance between the latter fixed contact and an adjacent fixed contact has the same value and is determined by the distance between this adjacent fixed contact on one hand and the switching contact connected with the point of junction via the transition impedance on the other hand, so that this distance which must be sufficient to insulate the voltage difference between the fixed contact appropriated for the connection to the tap having the lowest voltage of the regulating range and the fixed contact appropriated for the connection to the tap having the highest voltage of the regulating range is too large to insulate the voltage difference occurring between two adjacent fixed contacts and corresponding to the fine voltage steps of the transformer. That is why at a given number of fixed contacts arranged is the circular row the diameter of the known combined diverter switch tap selector of this kind is larger than is necessary for the required insulating distances, and conversely, a combined diverter switch tap selector of given diameter comprises less fixed contacts than is possible in connection with the insulating distances.

The invention has the object to provide a tapchanger comprising a combined diverter switch tap selector, in which more fixed contacts can be placed in a circular row of given diameter or a given number of fixed contacts can be arranged in a circular row of smaller diameter. The invention is to be seen in that at least one of the two contacts of the or each pair of adjacent fixed contacts, between which the voltage difference can become a multiple of the voltage of the fine voltage steps of the transformer, extends over a larger arc of a circle than the other fixed contacts and in that the rotatable switching contacts must be rotated for the changeover operation between the said or such a pair of fixed contacts through a greater angle than for the changeover operation between the other fixed contacts. in this combined diverter switch tap selector, in which the requirements for making the free distance between adjacent fixed contacts in all places in the diverter switch tap selector the same and thus having said free distances in all said places bridged by the constantly angularly spaced rotatable switching contacts inthe same way are met, which is important to ensure the most favorable changeover operation, the insulating distance between the set of rotatable switching contacts, when in the stationary position contacting a fixed contact, on one hand and anadjacent fixed contact on the other hand can be adapted to the voltage difference across said distance. This means that said insulating distance in the place, where said voltage difference is equal to the sum of all fine voltage steps, is greater than the insulating distance in the places, where the voltage difference is equal to one single fine voltage step, only and that, during the changeover operation from the lowest to the highest or from the highest to the lowest voltage of the regulating range of the fine voltage steps, the rotatable switching contacts must be rotated through a greater angle than during the changeover operation between two fixed contacts, between which the voltage difl'erence is equal to one single fine voltage step.

It is observed that a tap selector with a rotatable switching contact adapted to rotate through unequal angles has already been disclosed in the British Pat. specification No. 1,054,195. However, this specification relates to a selector switch which is designed to operate under no load conditions only and in which there is no bridging by a transition impedance between the fixed contact, so that therein the requirement for equal free distances between adjacent fixed contacts need not be met'and the problem of the insulating distance between a fixed contact and the closest switching contact connected via a transition impedance and forming part of a set of rotatable switching contacts being in the stationary position in engagement with an adjacent fixed contact does not occur.

The invention will be further elucidated with the aid of the drawing. In the drawing:

FIG. 1 is a diagrammatical view of a combined diverter switch tap selector for the fine voltage steps of usual construction provided with a set of three rotatable switching contacts,

FIG. 2 is a diagrammatical view of a combined diverter switch tap selector for the fine voltage steps constructed in accordance with the invention and provided with a set of three rotatable switching contacts,

' FIG. 3 is a diagrammatical view of a combined diverter switch tap selector constructed in accordance with the invention and provided with three rotatable switching contacts, said diverter switch tap selector being combined with a changeover selector for the coarse voltage steps of the transformer winding,

FIG. 4 is a diagrammatical view of a variant of the combined diverter switch tap selector for the fine voltage steps shown in FIG. 2 and provided with a set of two rotatable switching contacts only and FIG. 5 is a diagrammatical view of a variant of the combined diverter switch tap selector for the fine voltage steps shown in FIG. 3 and provided with a set of two rotatable switching contacts only.

In P10. 1 the reference numeral 1 designates the base winding and the reference numeral 2 designates the regulating winding permanently connected with said first winding of a regulating transformer. The regulating winding 2 is provided with taps 3, 4, 5, 6, 7, 8, 9, 10 for seven fine voltage steps. These taps are connected with the fixed contacts 11, 12, l3, 14, 15, 16, 17, 18 arranged in a circular row and forming part of a combined diverter switch tap selector for the fine voltage steps. A set of three mechanically rigidly switching contacts 19, 20, 21 mounted for rotation about the axis of said circular row cooperates with said fixed contacts. The middle one 20 of said switching contacts is directly and the outer ones 19, 20 thereof are, each one via a transition impedance 22, 23, connected with an annular point of junction 24. A network-line or a star-point conductor 25 is connected to said point of junction.24. The centers of the fixed contacts 11-18 are equiangularly spaced and said fixed contacts extend over equal arcs of a circle, so that the free distances between the fixed contacts are also equal. This is necessary to obtain a favorable bridging of two adjacent fixed contacts during the changeover operation.

It will be apparent that the greatest voltage difierence, that means a voltage equal to the entire voltage of the regulating winding 2, will occur between the fixed contacts 11 and 18, where as a voltage difference equal to one single fine voltage step only will occur between all other adjacent fixed contacts. This means, that the insulating distance indicated by the arrow 26, said insulating distance being adapted to withstand the great voltage difi'erence between the fixed contacts 1 1 and 18 and extending from the fixed contact ll'to the switching contact 19 which is found 011' the fixed contact 18, when the set of switching contacts 19, 20, 21 is in the shown stationary position, determines the diameter of the combined diverter switch tap selector. This insulating distance is much greater than necessary in all places, where not more than the voltage of one fine voltage step occurs between the switching contact 19 or 21 and a fixed contact. Consequently, the diameter of the diverter switch tap selector shown in FIG. 1 is too great in respect of the required insulating distances.

The combined diverter switch tap selector illustrated in FIG. 2, which is constructed in accordance with the invention, is also provided with eight fixed contacts 27, 28, 29, 30, 31, 32, 33, 34 arranged in a circular row and a set of three rotatable switching contacts 35, 36, 37 and two transition impedances 38, 39 cooperating therewith. Also in this case the free distances between the consecutive fixed contacts are equal. However, in this device the fixed contacts 27 and 34 connected to the ends 3 and of the regulating winding 2 extend each over a larger arc of a circle than the other fixed contacts 28-33 and the set of rotatable switching contacts 35, 36, 37 is, in its stationary position, asymmetrically positioned on the fixed contact 27 or 34, in such a manner, that the insulating distance 40 between the switching contact 35 and the fixed contact 27 or between the switching contact 37 and the fixed contact 34 sufiices just to insulate the great voltage difference between the fixed contacts 27 and 34. The insulating distance 41 between the switching contact 37 and the fixed contact 33, where a voltage difference equal to a single voltage step only obtains, is considerably smaller than the insulating distance 40 as appears from FIG. 2. This smaller insulating distance 41 is found in all stationary positions of the switching contacts 35, 36, 37, where the small voltage difference between the switching contact 35 or 37 and a fixed contact obtains. During the changeover operation between the fixed contacts 27 and 34 the set of switching contacts 35, 36, 37 is rotated through a greater angle than during the changeover operation between the other adjacent fixed contacts. The diameter of this diverter switch tap selector is for the same number of fixed contacts smaller than that of the diverter switch tap selector shown in FIG. 1.

In FIG. 3 the regulating winding 2 for the fine voltage steps is separated from the base winding 1. Two windings 42 and 43 for the coarse voltage regulation are permanently connected in series with the base winding 1. The combined diverter switch tap selector constructed in accordance with the invention is provided with eight fixed contacts 44, 45, 46, 47, 48, 49, 50, 51, of which the fixed contacts 44-50 are connected with the taps 4-10 of the regulating winding 2 and the fixed contact 51 is adapted to be optionally connected via the rotatable switching contact 52 and the fixed contacts 53, 54, 55 of a double changeover selector with the taps 56, 57, 58 at the ends of the windings 42, 43 for the coarse voltage regulation. The beginning 3 of the regulating winding 2 can also be connected, via the rotatable switching contact 59 which is mechanically rigidly coupled with the rotatable switching contact 52 and via the fixed contacts 53, 54, 55 of the double changeover selector, with the taps 56, 57, 58, so that the regulating range extends in a manner known per se with the aid of the double changeover selector and the combined diverter switch tap selector for the fine voltage steps over a range of 2X8+7ax23 voltage steps.

In the diverter switch tap selector constructed in accordance with the invention the fixed contacts 44, 50 and 51 extend over larger arcs of a circle than the fixed contacts 45, 46, 47, 48, 49 and the arc of the fixed contact 51 is still somewhat larger than that of each one of the fixed contacts 44 and 50. Also in this device the free distances between the fixed contacts 44-51 are equal. In the illustrated positions of the diverter switch tap selector and the changeover selector a voltage difference equal to the fine voltage step between the taps 3 and 4 of the regulating winding is found between the fixed contacts 44 and 51 of the diverter switch tap selector only, whereas the entire voltage of the regulating winding is found between the fixed contacts 50 and 51. If the diverter switch tap selector is changed over and if it is in the intermediate position, in which the rotatable switching contact 59 is still in contact with the fixed contact 53 and the tap 56 and the rotatable switching contact 52 is connected to the fixed contact 54 and the tap 57, the entire voltage of the regulating winding occurs between the fixed contacts 44 and 51 of the diverter switch tap selector, whereas only a voltage difference equal to one single fine voltage step is found between the fixed contacts 50 and 51, since the windings 42 and 43 each produce a voltage, which is one fine voltage step higher than the voltage across the entire regulating winding 2. The great voltage difierence thus appears alternately between the contacts 44 and 51 and between the contacts 50 and 51, so that the insulating distances 60 and 61, which are equal, must be capable to withstand this great voltage difierence. In all other places only a voltage difference equal to a single fine voltage step occurs, so that in these places the insulating distance 62 between a switching contact 63 or 65 of the set of rotatable switching contacts 63, 64, 65 and transition impedances 66, 67 which are in their stationary position in respect of a fixed contact on one hand and an adjacent fixed contact 44-50 on the other hand may be made considerably smaller. This results in the smaller arc of a circle of the fixed contacts 45, 46, 47, 48, 49 and in the difference between the larger arc of each one of the fixed contacts 44 and 50 and the still larger arc of the fixed contact 51. Also in this diverter switch tap selector the diameter for the same number of fixed contacts is smaller than that of the diverter switch tap selector shown in FIG. 1.

FIG. 4 shows an other combined diverter switch tap selector for the regulating circuit of connections shown in FIG. 2. Therein the set of rotatable switching contacts comprises two switching contacts 68, 69 only, of which the contact 68 is directly connected to the point of junction 24 and the contact 69 is connected to said point via a transition impedance 70. The network-line or the star-point conductor 25 is connected to said point of junction 24. The device is provided with eight fixed contacts, which follow each other at equal free distances. The fixed contact 71 which must be connected with the end of the regulating winding extends over a larger arc than the other seven equal fixed contacts. If the rotatable switching contacts 68, 69 are in the shown stationary position on the fixed contact 71, the insulating distance 72 to the fixed contact 73, which is to be connected to the beginning of the regulating winding, is great and adapted to a voltage difference equal to seven time the fine voltage step. In all other stationary positions 68', 69', of which one is shown in dotted lines, the smallest insulating distance 74 between a rotatable switching contact and an adjacent fixed contact is much smaller, which is possible, as the voltage difierence which must be insulated is equal to one fine voltage step only.

The combined diverter switch tap selector shown in FIG. 5 may be used instead of the device illustrated in FIG. 3 in the circuit of connections shown therein. In FIG. 5 the diverter switch tap selector is provided with a set of two rotatable switching contacts 75, 76 only, of which the contact 75 is directly connected and the contact 76 is connected via the transition impedance 77 to the point of junction 24. The device comprises eight fixed contacts which lie pairwise at equal distances from each other. The fixed contact 78 connected to the changeover selector 52, 53, 54, 55, 59 and the adjacent fixed contact 79 extend over a larger arc of a circle than the other fixed contacts. If the rotatable switching contacts 75, 76 are in the shown stationary positions 75', 76'; 75, 76; 75", 76" on the consecutive fixed contacts 79, 78, 80, the insulating distance 81, 82 between a rotatable switching contact and an adjacent fixedcontact, the voltage of which can differ a multiple of a single fine voltage step from that of said switching contact, is great. If, however, the voltage difference between a rotatable switching contact and an adjacent fixed contact can not become greater than the voltage of one single fine voltage step the insulating distance need not be greater than-that indicated by the arrow 83.

The combined diverter switch tap selectors (FIGS. 4 and 5) having a set of two rotatable switching contacts only may be provided with one or two fixed contact only, which extend over a larger are of a circle, whereas the diverter switch tap selectors (FIGS. 2 and 3) havinga set of three rotatable switching contacts require two or threefixed contacts which extend over larger arcs of a circle. That is why the diverter switch tap selectors with the asymmetrical construction (FIGS. 4 and 5) can have a still smaller diameter than the diverter switch tap selectors with the symmetrical construction (FIGS. 2 and 3).

What I claim is:

I. An on-load tapchanger for a regulating transformer having a regulating winding provided with a series of taps comprising;

a selector-switch for fine voltage steps, said switch including a circular row of fixed contacts for connection with the taps of said regulating winding, at least two rigidly coupled but angularly spaced rotatable switching contacts ,mounted for stepwise rotation about the axis of said circular row of fixed contacts and cooperating with said fixed contacts, said set of rotatable switching contacts being adapted to be rotated stepwise through an unlimited number of revolutions in either direction with at least one of said rotatable contacts engaging a fixed contact in each stepwise position, a terminal point, and at least one transition impedance, one of said rotatable switching contact being directly connected to said terminal point and the'other rotatable switching contact being connected through said transition impedance with said terminal point, said fixed contacts, said rotatable switching contacts, said transition impedance and said terminal point constituting together one phase of said selector-switch, the angular spacing between said fixed contacts being equal and the angular spacing between said rotatable switching contacts being such that they bridge across a pair of adjacent fixed contacts during their movement from one stepwise position to the next, at least one of the two contacts of a pair of adjacent fixed contacts, between which the voltage difference can become a multiple of the voltage of the fine voltage steps of the regulating winding of the transformer, extending over a larger arc of a circle than the other fixed contacts and the set of rotatable switching contacts having only one angular rest position when it is in contact with said fixed contact extending over a larger arc of a circle, so that the angular step of the set of rotatable switching contacts necessary for the changeover operation between the fixed contacts of the said pair of adjacent fixed contacts is greater than the angular step thereof necessary for the changeover operation between the fixed contacts of each other pair of adjacent fixed contacts.

2. In an on-load tapchanger for a regulating transformer having a series of regulating taps at which progressively greater voltages may appear;

a circular row of equiangularly spaced fixed contacts equal to the number of taps and connected successively to respective taps whereby one pair of adjacent contacts may display maximum voltage therebetween;

rotatable contact means including a pair of angularly spaced rotatable contacts mounted for stepwise rotation about the axis of said circular row of fixed contacts, the angular spacing between said rotatable contacts being sufficiently great to bridge across each pair of adjacent fixed contacts during movement thereof from one stepwise position to the next and the stepwise positions of said rotatable contacts being such as to position one of them at a particular point on an associated fixed contact at each stepwise position;

a terminal point;

connector means connecting said rotatable contacts with said terminal point including means for limiting current flow when said rotatable contacts are in a bridging relation as aforesaid;

said angular spacing between fixed contacts being sufficiently great to insulate between said one pair of adjacent fixed contacts between which said maximum voltage may occur, the angular extent of at least one of the fixed contacts of said one pair thereof being greater than the angular extents of at least the majority of the remainder of the fixed contacts whereby to permit the diameter of said circular row to be smaller than otherwise permitted; and

the angular extent of rotatable contact stepwise movement between said one pair of fixed contacts being greater than the angular extent of rotatable contact stepwise movement between adjacent ones of said majority of the remainder of the fixed contacts.

3. In an on-load tapchanger as defined in claim 2 wherein said rotatable contact means includes a third rotatable contact, said connector means including first and second transition impedances, said rotatable contacts being equiangularly spaced and the outer two of said rotatable contacts being connected to said 'tenninal point through said first and second transition impedances respectively.

4.'In an on-load tapchanger as defined in claim 3 wherein both of said one pair of fixed contacts are of equal angular extent.

5. In an on-load tapchanger as defined in claim 4 wherein a third fixed contact adjacent said one pair thereof is of said greater angular extent, the rotatable contact stepwise movements between the three fixed contacts constituted by said one pair and said third contact being equal.

6. In an on-load tapchanger as defined in claim 2 wherein both of said one pair of fixed contacts are of equal angular extent.

7. In an on-load tapchanger as defined in claim 6 wherein a third fixed contact adjacent said one pair thereof is of said greater angular extent, the rotatable contact stepwise movements between the three fixed contacts constituted by said one pair and said third contact being equal.

8. In an on-load tapchanger for a regulating transformer provided with a series n of taps at which progressively greater voltages may appear, a selector switch which comprises, in combination;

a series of fixed contacts equal to said number n of taps, said fixed contacts being arranged in a circular row of radius d so that a first fixed contact is connected with the highest voltage tap and an adjacent second fixed contact is connected to the lowest voltage tap so that a maximum voltage differential may be impressed between that one pair of fixed contacts constituted by said first and second fixed contacts, all of said fixed contacts being separated at least by a circumferential spacing a between each adjacent pair of fixed contacts which is sufficient to prevent arcing between said first and second fixed contacts when said maximum voltage is impressed therebetween;

rotatable contact means mounted for stepwise rotation about the center of said circular row, said rotatable contact means comprising a plurality of fixedly separated rotatable contacts having a maximum circumferential spacing which is at least equal to said distance a whereby,

to bridge between adjacent fixed contacts during each stepwise movement of the rotatable contact means;

a terminal point;

means connecting said rotatable contact means with said terminal point including means for limiting current flow between said terminal point and any adjacent pair of fixed contacts when the latter are bridged as aforesaid during a stepwise movement of the rotatable contact means;

at least some of said fixed contacts, other than said first and second fixed contacts, being of circumferential lengths less than said maximum spacing between said plurality of rotatable contacts whereby to reduce said diameter d, and

at least said first fixed contact being of greater circumferential length than said some fixed contacts; and

the stepwise position of said rotatable contact means at said first fixed contact being such as to engage two rotatable being connected sequentially with individual ones of said taps so that a first fixed contact located at said first rest position and a second fixed contact located at said second rest position may display the greatest voltage difference contacts therewith without diminishing the insulating which may exist due to the progressively increasing voltspace between said first and second fixed contacts, the ages at said taps, whereas pairs of those fixed contacts stepwise positions of said rotatable contact means at said located at those rest positions which are separated by said some fixed contacts being such as to engage at least one one angular extent of stepwise movement may display rotatable contact therewith while positioning another only a fraction of said greatest voltage difference rotatable contact out ofengagement therewith but spaced 10 the ebe e al of d fi ed onta s be ng cu from a next adjacent fixed contact by an amount sufiiferentially elongate and each being separated from those cient to prevent arcing thereto, the angular extent of the On either side of it by amounts sufiicient to prevent elecstepwise movement between said fir t and e nd fi d trical breakdown therebetween due to voltage differences contacts being greater than the angular extent of stepwise imposed by said taps; movement between any adjacent pair of said some fixed a lemma! P contacts. said rotatable contact means including at least first and 9. in an on-load tapchanger for a regulating transformer rotatable cqmacts disposed filfed circumhaving a series f taps at hi h progressively greater voltages ferential spaced relation to each other which Is of amount may appear; suflicient to bridge between each pair of adjacent fixed rotatable contact means mounted on a predetermined axis during a stepwise mQVemem between fest P for making sequential stepwise movements between a Ions; number of rest positions equal to the number of said taps, meals q p s Said mimics 531d lemma] most of said stepwise movements being of one angular ex- P l mcluflmg means for llmltlng P l a tent and at least one stepwise movement between a first P of afilacent fixed Contact) and l {ermmal IP rest position and a second rest position being of an angusald rotatable coma? means 15 a Posmon lar extent greater than said one angular extent whereby Pndgmg such P afqresald; and two diff angular movements are involved in making said first fixed contact being of greater arcuate extent than a complete rotation f id rotatable Contact means; those fixed contacts which display fractional voltage difa circular row of fixed contacts centered about said axis, ferences respect fixed m slde said fi d contacts being equal to said number of taps and thereof, whereby the diameter of said circular row is less located at said rest positions so as to be engaged selectivethan would otherwlse be requu'ed' ly by said rotatable contact means, said fixed contacts 

1. An on-load tapchanger for a regulating transformer having a regulating winding provided with a series of taps comprising; a selector-switch for fine voltage steps, said switch including a circular row of fixed contacts for connection with the taps of said regulating windIng, at least two rigidly coupled but angularly spaced rotatable switching contacts mounted for stepwise rotation about the axis of said circular row of fixed contacts and cooperating with said fixed contacts, said set of rotatable switching contacts being adapted to be rotated stepwise through an unlimited number of revolutions in either direction with at least one of said rotatable contacts engaging a fixed contact in each stepwise position, a terminal point, and at least one transition impedance, one of said rotatable switching contact being directly connected to said terminal point and the other rotatable switching contact being connected through said transition impedance with said terminal point, said fixed contacts, said rotatable switching contacts, said transition impedance and said terminal point constituting together one phase of said selector-switch, the angular spacing between said fixed contacts being equal and the angular spacing between said rotatable switching contacts being such that they bridge across a pair of adjacent fixed contacts during their movement from one stepwise position to the next, at least one of the two contacts of a pair of adjacent fixed contacts, between which the voltage difference can become a multiple of the voltage of the fine voltage steps of the regulating winding of the transformer, extending over a larger arc of a circle than the other fixed contacts and the set of rotatable switching contacts having only one angular rest position when it is in contact with said fixed contact extending over a larger arc of a circle, so that the angular step of the set of rotatable switching contacts necessary for the changeover operation between the fixed contacts of the said pair of adjacent fixed contacts is greater than the angular step thereof necessary for the changeover operation between the fixed contacts of each other pair of adjacent fixed contacts.
 2. In an on-load tapchanger for a regulating transformer having a series of regulating taps at which progressively greater voltages may appear; a circular row of equiangularly spaced fixed contacts equal to the number of taps and connected successively to respective taps whereby one pair of adjacent contacts may display maximum voltage therebetween; rotatable contact means including a pair of angularly spaced rotatable contacts mounted for stepwise rotation about the axis of said circular row of fixed contacts, the angular spacing between said rotatable contacts being sufficiently great to bridge across each pair of adjacent fixed contacts during movement thereof from one stepwise position to the next and the stepwise positions of said rotatable contacts being such as to position one of them at a particular point on an associated fixed contact at each stepwise position; a terminal point; connector means connecting said rotatable contacts with said terminal point including means for limiting current flow when said rotatable contacts are in a bridging relation as aforesaid; said angular spacing between fixed contacts being sufficiently great to insulate between said one pair of adjacent fixed contacts between which said maximum voltage may occur, the angular extent of at least one of the fixed contacts of said one pair thereof being greater than the angular extents of at least the majority of the remainder of the fixed contacts whereby to permit the diameter of said circular row to be smaller than otherwise permitted; and the angular extent of rotatable contact stepwise movement between said one pair of fixed contacts being greater than the angular extent of rotatable contact stepwise movement between adjacent ones of said majority of the remainder of the fixed contacts.
 3. In an on-load tapchanger as defined in claim 2 wherein said rotatable contact means includes a third rotatable contact, said connector means including first and second transition impedances, said rotatable contacts being equiangularly spaced and the outer two of said rotatable contacts being conneCted to said terminal point through said first and second transition impedances respectively.
 4. In an on-load tapchanger as defined in claim 3 wherein both of said one pair of fixed contacts are of equal angular extent.
 5. In an on-load tapchanger as defined in claim 4 wherein a third fixed contact adjacent said one pair thereof is of said greater angular extent, the rotatable contact stepwise movements between the three fixed contacts constituted by said one pair and said third contact being equal.
 6. In an on-load tapchanger as defined in claim 2 wherein both of said one pair of fixed contacts are of equal angular extent.
 7. In an on-load tapchanger as defined in claim 6 wherein a third fixed contact adjacent said one pair thereof is of said greater angular extent, the rotatable contact stepwise movements between the three fixed contacts constituted by said one pair and said third contact being equal.
 8. In an on-load tapchanger for a regulating transformer provided with a series n of taps at which progressively greater voltages may appear, a selector switch which comprises, in combination; a series of fixed contacts equal to said number n of taps, said fixed contacts being arranged in a circular row of radius d so that a first fixed contact is connected with the highest voltage tap and an adjacent second fixed contact is connected to the lowest voltage tap so that a maximum voltage differential may be impressed between that one pair of fixed contacts constituted by said first and second fixed contacts, all of said fixed contacts being separated at least by a circumferential spacing a between each adjacent pair of fixed contacts which is sufficient to prevent arcing between said first and second fixed contacts when said maximum voltage is impressed therebetween; rotatable contact means mounted for stepwise rotation about the center of said circular row, said rotatable contact means comprising a plurality of fixedly separated rotatable contacts having a maximum circumferential spacing which is at least equal to said distance a whereby to bridge between adjacent fixed contacts during each stepwise movement of the rotatable contact means; a terminal point; means connecting said rotatable contact means with said terminal point including means for limiting current flow between said terminal point and any adjacent pair of fixed contacts when the latter are bridged as aforesaid during a stepwise movement of the rotatable contact means; at least some of said fixed contacts, other than said first and second fixed contacts, being of circumferential lengths less than said maximum spacing between said plurality of rotatable contacts whereby to reduce said diameter d, and at least said first fixed contact being of greater circumferential length than said some fixed contacts; and the stepwise position of said rotatable contact means at said first fixed contact being such as to engage two rotatable contacts therewith without diminishing the insulating space between said first and second fixed contacts, the stepwise positions of said rotatable contact means at said some fixed contacts being such as to engage at least one rotatable contact therewith while positioning another rotatable contact out of engagement therewith but spaced from a next adjacent fixed contact by an amount sufficient to prevent arcing thereto, the angular extent of the stepwise movement between said first and second fixed contacts being greater than the angular extent of stepwise movement between any adjacent pair of said some fixed contacts.
 9. In an on-load tapchanger for a regulating transformer having a series of taps at which progressively greater voltages may appear; rotatable contact means mounted on a predetermined axis for making sequential stepwise movements between a number of rest positions equal to the number of said taps, most of said stepwise movements being of one angular extent and at least one stepwise movement Between a first rest position and a second rest position being of an angular extent greater than said one angular extent whereby two different angular movements are involved in making a complete rotation of said rotatable contact means; a circular row of fixed contacts centered about said axis, said fixed contacts being equal to said number of taps and located at said rest positions so as to be engaged selectively by said rotatable contact means, said fixed contacts being connected sequentially with individual ones of said taps so that a first fixed contact located at said first rest position and a second fixed contact located at said second rest position may display the greatest voltage difference which may exist due to the progressively increasing voltages at said taps, whereas pairs of those fixed contacts located at those rest positions which are separated by said one angular extent of stepwise movement may display only a fraction of said greatest voltage difference therebetween, all of said fixed contacts being circumferentially elongate and each being separated from those on either side of it by amounts sufficient to prevent electrical breakdown therebetween due to voltage differences imposed by said taps; a terminal point; said rotatable contact means including at least first and second rotatable contacts disposed in fixed circumferential spaced relation to each other which is of amount sufficient to bridge between each pair of adjacent fixed contacts during a stepwise movement between rest positions; means connecting said rotatable contacts to said terminal point including means for limiting current flow between a pair of adjacent fixed contacts and said terminal point when said rotatable contact means is in a position bridging such pair as aforesaid; and said first fixed contact being of greater arcuate extent than those fixed contacts which display fractional voltage differences with respect to fixed contacts on either side thereof, whereby the diameter of said circular row is less than would otherwise be required. 